Transmission system



Filed Oct. 30, 1926 3 Sheets-Sheet 1 a g ziw M e w 7 M a 0 m w a m m w \Qdm 55 m 55 B 5 W M 5 .Mcb. w 5 .MM. 0 m Z w n w m 4 mrenfor; A/lm C fi/tkiesan June 10, 1930. A. c. DICKIESON TRANSMISSION SYTEM 3 Sheets-Sheet 2 Filed Oct. 30, 1926 [BM/2,0700 Pol/12 Fig.5

lure/var.- 4/700 6 fi/tk/ksan Alia/my June 10; 1930. Dl Kl s N 1,762,768

TRANSMISSION SYSTEM Filed Oct. 50, 1926 3 Sheets-Sheet 3 numb HERE" mnm \ 27-1 I 32 Iii F A l l l' I 29 a LI mum /m enfor.- A/fan C. 0/8/0250 9 Alramey Patented June 10, 193% nurse ALTON C. DIC'KIESON, OF BROOKLYN, NEW YORK, ASSIGNOR TO BELL TELEPHONE LAB- ORATORIES INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK TRANSMISSION SYSTEM Application filed October 30, 1926. Serial No. 145,170.

This invention relates to transmission sys tems and is especially applicable to electrical signaling. I

An object of the invention is to increase the sensitivity and precision of electrical response circuits.

Another object of the invention is to prevent crosstalk and other extraneous noise from passing through a repeater.'

3o A specific object of the invention is to reduce the transmission in a given direction of signals which are transmitted through a repeater in :the opposite direction, thereby reducing echo efiects.

1e In carrying out these objects there is provided a sensitive response circuit which is widely useful in the electrical arts, for example to control the application of current to an electrical circuit.

2e The response circuit in its preferred form comprises a s ace discharge tube rectifier having the usua grid, filament and plate electrodes. The .grid of the tube is made so strongly negative that very little or no current flows in the plate circuit and the filament-plate resistance of the tube is therefore very high or infinite. Under such condition slight changes in the voltage impressed upon the grid of the tube produce large changes n as the filament-plate resistance. Great sensitivity is thus obtained by confining the range of operation to the lowermost portion of the input voltage-output current characteristic of the tube. Moreover, it has been found that over the larger portion of this range changes in the filament-plate resistance of the tube are substantially proportional to changes 1n the grid potential.

According tothe invention in one aspect, the plate circuit of the rectifier tube of the arrangement just described is included in one arm of a 'Whea-tstone bridge which may be normally balanced or normally unbalanced, depending upon the purpose for which it is used, and the condition of the bridge is varied in response to changesin the grid potential of the rectifier tube.

The invention in an important aspect is the control of a space discharge tube repeater by the sensitive response circuit to render the former responsive or non-responsive to incoming signals. The plate circuit resistance of the rectifier tube is made high enough to unbalance the Wheatstone bridge when no signals are being received by the repeater. When the bridge is thus unbalanced, a negative biasing potential is impressed upon the grid of the repeater tube, which is strong enough to block this tube. When signals are received at therepeater, however, a portion of the resulting voltage is impressed upon the grid of the rectifier tube and tends to lower' the resistance of its plate circuit. This action restores the bridge to a partial balance and reduces the biasing potential upon the grid of the repeater tube to the point where the repeater is operated to transmit the signals.

The invention in the aspect just mentioned is particularly applicable to two-way repeaters having two oppositely directed channels to repeat signals in two directions. Such repeaters usually rely upon a condition of balance between the transmission line sections and artificial balancing circuits to prevent currents which are transmitted through one channel from leaking back through the oppositely directed channel. It is very diificult to v maintain this condition of balance, and as a 7 result there are heard echoes which lag slight-,

ly behind the original voice currents and produce an unpleasant effect upon the subscribers ear. This condition of unbalance may cause voice currents to circulate around the repeater, producing singing and tending to distort the amplified signals. Another example of echoes which are particularly sitely directed channel. In the case of echoes due to unbalance, the leakage currents are merely dissipated in the idle channel. For the purpose of overcoming echoes caused by reflection beyond the repeater each channel is sometimes equipped with a time :delay circuit to render the channel operative a certain time after currents have ceased to flow in the opposite channel. In order to preventreflected currents from passing back through the repeater to the transmitting station, it is necessary to time the delay circuits of each repeater with great care, depending upon the length of the line over which reflection occurs. It is also well known that such repeaters will amplify crosstalk and other extraneous noise, as well as the ordinary voice currents. provides a simple and eflicicnt noise and echo 20 suppressor which may be used with different transmission lines without special adjustment. Inioperation, echoes and other noise effects on the line are too weak to render this repeater operative, the signaling currents betion with the accompanying drawings, in which similar reference characters designate corresponding parts in the several views, and in which:

Fig. 1 is a diagrammatic illustration of a sensitive response circuitembodying the invention;

Figs. 2, 3 and 4: show curves used in describing the operation of the response circuit of Fig. 1;

Fig. 5 is a diagrammatic illustration of a two-Way repeater employing the noise and echo suppressor of the invention;

Fig. 6 is a similar View of a two-way repeater employing a modified noise and echo suppressor; and

Fig. 7 is a diagran'imatic illustration of a circuit arranged to repeat speech and ringing currents.

The sensitive response circuit shown in Fig. 1 is coupled to an incoming circuit 10 by means of a transformer 11, and is coupled to an outgoing circuit 12 by means of a transformer 13. The response circuit includes a three-electrode space discharge tube rectifier 14 having its input circuit connected to the secondary winding of the transformer 11 and its output circuit forming one arm of a Vv'heatstone bridge 15. provided with the usual sources of energy 16, 17 and 18 to properly polarize the grid electrode, to heat the cathode, and to furnish anode-cathode current, respectively. Space current from the source 18 is supplied to the rectifier through a choke coil 19. A condenser 20 prevents this current from flowing through the bri ige.

The. Wheatstone bridge includes a source 65 of sustained oscillations 21 adapted to imweaves The present invention, however,

in sufliciently strong to operate the repeater. he lnventlon will be described 1n connec- The rectifier 14 is press current on the outgoing circuit 12 only when the bridge is unbalanced. The constants of the bridge are so chosen that the bridge is balanced when a predetermined voltage is impressed upon the incoming cm cuit 10, so that no current from the source 21 will be impressed upon the outgoing circuit 12 at such time. In balancing thebridge, the grid or control electrode of the rectifiertube 14 is made so strongly negative that largechanges in the filament-plate resistance result from minute changes in the grid potential. Accordingly, when an increased voltage is impressed 'upon the grid circuit of the tube through the transformer 11, a substanso tial change is produced in the filament-plate resistance and the bridge becomes unba'l anced.

In the simple application of the invention shown in Fig. 1, the sensitive response .circuit may be utilized for testing purposes. A source of Waves to be tested, say of 5,000 cycles frequency, may be connected to the circuit 10 in any suitable manner. The sourceof oscillations 21 may be arranged to produce waves of, say 200 cycles frequency. When a predetermined voltage is impressed upon the circuit 10 the bridge 15 is balanced and no current flows from the source 21 through the transformer 13. When the volt- ()5 age impressed upon the circuit 10 increases, however, the filament-plate resistance of the rectifier tube 14 decreases, as hereinafter described, and the Wheatstone bridge is uubalanced, permitting current of 200 cycles frequency to flow through the transformer 13. The current which is thus impressed upon the outgoing circuit 12 is selectively transmitted through the low pass filter LP and impressed upon the detector circuit D, which may 0011- tain a galvanometer. The low pass filter LP suppresses currents of all frequencies higher than those produced by the source 21, so that the detector D is not affected by the higher frequency currents which are present in the no output circuit of the rectifier. The reading of the detector circuit D serves as an indication of the current which is impressed upon the incoming circuit 10.

The operation of the rectifier tube is illustrated by the curves of Fig. 2, in which the ordinates represent the plate current and the abscissae represent the grid volta e of the tube. As shownby these curves, tie initial voltage impressed upon the grid of the recti- 12o fier tube by the source 16 is so strongly negatwo that no plate current flows in the output circuit of the rectifier. Under such con dition, the rectifier tube impedance is in finite. The curve A represents variations in voltage which are impressed upon the grid of the rectifier tube through the transformer 11. Only voltage amplitudes which are strong enough to carry the grid potential tothe right hand side of theline indicating t extinct'ion point cause plate current to flow in the output circuit of the rectifier. The shaded portions of this curve may comprise only a small percentage of the impressed wave and still produce a large change in the impedance of the rectifier tube.

The operation of the sensitive response circu-it is further illustrated by the curve of Fig. 3 in which the ordinates represent the filament-plate resistance of the rectifier tube and the abscissae represent the grid voltage. It will be seen from this curve that the filament-plate resistance of the rectifier tube changes from 4,000 ohms at zero grid potential to about 60 000 ohms at minus 25 volts grid potential. lf the three simple resistance arms of the Wheatstone bridge shown in Fig. 1 are given a large value, such as 60,000 ohms apiece, and the grid battery 16 is made so strongly negative that the bridge is balanced, then no current flows from the source 21 through the transformer 13 but a small increase in the potential which is impressed upon the grid of the rectifier tube through the transformer 11 will produce a relatively large change in the filament-plate resistance of the tube, thus throwing the bridge olf balance and permitting a large voltage to be impressed upon the transformer 13. Great sensitivity isobtained by confining the operation to the substantially straight portion of the characteristic curve.

- Fig. 4: illustrates graphically the change produced in the output voltage E across the primary winding of the transformer 13 in response to slight changes in the voltage E, applied to the grid of the rectifier tube. The

circuit of Fig.- 1 is arranged to detect small changes in the voltage E applied to the grid of the rectifier tube 14. The value of the negative" grid biasing potential supplied by the source 16 is so chosen that the plate resistance of the rectifier tube is such as to bal ance the Wheatstone bridge. In the curve of Fig. 4 the normal negative biasing potential is 25 volts. When the bridge is balancedthe voltage E is equal to Zero. if the potential impressed upon the grid of therectifier is varied, a change is also produced in the filament-plate resistance, unbalancing thebridge and causing the voltage E to increase. In other words, as the voltage E is increased a larger percentage of the voltage (2 is impressed upon the primary winding of transformer 18. This change may be read from the curve of Fig. l. The total voltage impressed upon the grid of the rectifier tube may be very large, but still a change as small as .1 volt will produce a 100% change in the voltage E. The sensitive response circuit may also be used for other purposes in the electrical arts.

In Fig. 5 the sensitive response circuit is utilized to reduce echoes and singing effects in a two-way telephone repeater, and also to prevent crosstalk and other extraneous noise from passing through the repeater. The repeater comprises two one-way repeating channels RVV and RE which are connected to transmission line sections lV and 1 by means of hybrid coilsH and H and assi'icial'ed balancing networks N and N. The channel RW includesv a three-electrode space discharge tube amplifier and the channel i216 includes a similar amplifier 26.

Voice currents which are received from the line section \V pass through the hybrid coil H into the channel RV? where they are amplified by the amplifier 25 and impressed through hybrid coil H upon line section ll. Similarly, voice currents received from the line section E pass through hybrid coil H into the channel RE and after being amplified in the amplifier 26 are transmitted through hybrid coil H to line section W'.

The undesirable noise and echo ell'ects mentioned above are overcome by means of this invention by rendering the space discharge tube amplifiers 25 and 26 operative only in response to voice currents of predetermined strength which are received from the line sections N and .E, respectively. The undesirable echoes, crosstalk and other noise eiiects which, of course, are below the transmission level of the voice currents, are inefiectual to operatethe repeater. Each of the channels RW and RE is provided with a branch circuit for controlling the operation of the repeater, but since these circuits are identical, only the circuit associated with the channel RW will be described in detail.

The branch circuit includes the sensitive response circuit, described above, which is coupled by means of the transformer 11 to the output of an amplifier A which has its input circuit connected across the conductors of the channel RW at a point between the hybrid coil H and the amplifier 25. The VVheatstone bridge includes a battery Ewhich supplies space current to the rectifier tube 14 and isalso arranged to supply a negative biasing potential to the grid of the amplifier 25 through a choke coil 27. The condenser '28 prevents this battery from being shortcircuited. The constants of the circuit are so chosen that the bridge is normally unbalanced and under such condition the biasing potential impressed upon the grid of the amplifier tube 25 from the battery 1* is so strong that no current llows in the output circuit of this tube. The circuit is so arranged that incoming currents above a predetermined transmission level which are impressed upon the input circuit of the rectifier 14 will tend to balance the bridge and reduce the negative biasing potential upon the grid of the amplifier tube 25 to the point where this tube will be operative to amplify the incoming signals. The condenser 33 provides a low impedance path for the alternating curits may berepresented by the resistance P shown in dotted lines, and the grid circuit of the repeater tube 25 may be represented by a resistance G. For the sake of simplicity it assumed. that the resistance arms R and R are equal and of a value which depends upon the desired value of the filament-plate resistance l By applying Kirchofi s laws to these circuits the following equations are obtained:

E=Z1P +1 13, (1 r assign; 2 ne ewanew-nu e (a) The term equal to the voltage impressed upon the grid of the repeater tube 25, that is, the grid biasing potential, which may be termed E /Vhen the grid of the repeater tube is negative, or non-conducting, the value of practically infinite and L is negligible. Therefore, approximately, 21 E =0. Substituting in equation 3) and rearranging,

The 'il 'heatstone bridge is balanced when equals L, which occurs when P equals it and at such time E equals zero That is, no voltage is impressed upon the grid of the repeater tube 25 at such time. If P is large compared with R (corresponding to the condition of no input current impressed on the rectifier tube}, then T is large compared with L. 'lically equal to l t and substantially half oi voltage impressed upon the grid of the repeater tuhe as a negative grid bias. This last is the condition of the circuit when no current is hein impressed upon the input circuit the rectifier 14:. However, when voice currents -impressed upon'the grid get i. s s c of the rectifier, the filament-plate resistance P decreases in value, tending to approach the limiting value 1 3 Thus L increases in value and l l de reases. a result of this action the negative grid biasing poten- E decreases in value until it reaches the normal 'operatiu ri'i potential for the u a :5 a) tube The constants of the circuit are so chosen that P never equals R and hence never reaches zero but approaches the correct grid potential for this tube as the In such case E is prac- 4 voltage impressed upon the rectifier tube increases.

The repeater circuit shown in Fig. 6 is similar to thatshown in Fig. 5 except that the sensitiveresponse circuit comprises a voice operated relay 29 having a contact device 30, this relay being of the type disclosed in U. S. Patent No. 1,579,887 to E. J. Pratt; issued April 6,1926. This specific bridge circuit is disclosed and claimed in my copending application Serial No. 145,171, filed Uctober 30,

' 1926. The contact device 30 is included with the resistance P in one arm if the Wheatstone bridge, the contact resistance being represented by the dotted line P. The contact resistance characteristics are such that the resistance remains substantially constant for input voltages within a certain definite range,

while above this range the resistance increases rapidly. Applying Kirchofis laws to these circuits, and assuming the resistance arms to be equal the following equations are obtained:

'E=I R,,+I (P+-.P (5 E I R +Z R1 I2R I1R +QI3IE +Z3G G As before, let E represent the voltage impressed upon the grid of the repeater tube 25. Then, when G is infinite and the term 21 E, is equal to zero.

E5522, not a When no voice currents are impressed upon the windings of relay 29 the contacts are closed and P equals zero. The resistance P is chosen smaller than R so that L is large compared with L. That is, alarge part of the voltage E is impressed upon the grid of the repeater tube 25 as a biasing potential. l/Vhen voice currents are impressed on this relay and it begins to operate'the resistance (Pl-P increases and tends to approach the limiting value R Then 1 decreases and E also decreases until it reaches the normal operating grid bias.

Fig. 7 shows a simple embodiment of the nel RW the relay 29 is operated. and the potential on the grid of the amplifier 25 is reduced in the manner described above, so that space current flows in the output circuit of the amplifier. During the 20 cycle interruptions in the ringing current, however, the relay 29 does not operate and hence no space current flows in the output circuit of the .amplifier 25. As a result of this continued action the space current alternately rises and then falls to zero in, response to the interrupted ringing currents, thereby producing a 20 cycle current in the output circuit. This current actuates an alternating current relay 81, which is connected in the output circuit of the amplifier and tuned to this frequency. The operation of the relay 31 connects a source of interrupted ringing currents 32 to the outgoing circuit. If desired, a slow releasing relay or other suitable device may be associated with the relay 31 to prevent the possibility of having this relay operate in response to interrupted voice currents.

The invention is also capable of many other modifications and adaptations which differ in detail from those herein shown and described but which are included within the scope of the appended claims.

What is claimed is:

1. A sensitive response circuitcomprising a space discharge tube having an anode, a cathode and a grid, a lVheatstone bridge having an arm containing the cathode-anode resistance of said tube, a source of periodically varying potential connected between two opposite terminals of said bridge, and means for impressing a negative biasing potential upon said grid such that small variations in the grid potential produce substantially maximum changes insaid resistance.

2. A sensitive response circuit comprising a space discharge tube having an anode, cathode and grid, a Wheatstone bridge having an arm containing the cathode-anode resistance of said tube, a source of potential connected between two opposite terminals of said bridge, a circuit in shunt to said cathodeanode-resistance containing a second source of potential and a high. impedance, means for preventing current from said second source from flowing in the other arms of said VVheatstone brid 'e and means for impressing a biasing potential upon said grid so strongly negative that small variations in grid potential produce large changes in said cathode-anode resistance.

3. A transmission system comprising a circuit over which waves are transmitted, a

space discharge amplifier including a grid for repeating said waves, means for impressing a large biasing potential upon said grid to block said amplifier, and means responsive to waves above a certain amplitude to reduce said biasing potential to render said amplifier operative.

4. A transmission system comprising a circuit over which waves are transmitted, a space discharge amplifier including a grid for repeating said waves, a source of negative biasing potential for said grid, means for rendering a large part of said potential efiective to block said amplifier, and means responsive to Waves above a certain amplitude to geduce the potential impressed upon said gri 5. A transmission system comprising a circuit over which waves are transmitted, a space discharge amplifier for repeating said waves, means comprising a source of biasing voltage for blocking said amplifier, a rectifier for producing a direct current voltage in response to incoming waves, and means to oppose said voltage to the biasing voltage to unblock said amplifier.

6. A transmission system comprising a circuit over which waves are transmitted, a space discharge amplifier including a grid for repeating said waves, means for impressing a large biasing potential upon said grid to block said amplifier, and a rectifier responsive to incoming waves to reduce said biasingpotential to render said amplifier operative.

7. A transmission system comprising a circuit over which waves are transmitted, a

rectifier responsive to incoming waves for controlling the resistance of said bridge, the resistance changes of said bridge varying the biasing potential impressed upon said grid.

9. A transmission system comprising a circuit over which waves are transmitted, a space discharge amplifier including a grid for repeating said waves, a Wheatstone bridge including a source of negative biasingpotential effective to bias said grid, a space discharge device having an ano e, a cathode and a grid and having its cathode-anode resistance in one arm of said bridge, and means for impressing incoming waves upon said device said waves acting to vary said resistance, the resistance variations operating to vary the biasin potential impressed upon the grid of sai amplifier.

10. 'A repeating device having an inputoutput characteristic whichis curved in part and substantially straight in part, a source of current in the output circuit of said repeater, a source of potential associated with i and connected to said mcomin the input circuit of said repeater for normallyblocking it to prevent flow of current through said repeater from said output source, a source of waves to be repeated, and means responsive to said waves to unblock said repeater and cause it to repeat said waves without substantial distortion by utilizing the substantially straight portion of the input-output characteristic.

11. A space discharge repeater comprislng an output circuit having a source of space current, and an input circuit, said repeater having an input-output characteristic which is in part substantially straight and in part curved, a source of sets of waves of different amplitude respectively, means for impress.-' ing all of said waves upon said repeater, a source of potential for normally blocking'the flow of current in said output circuit, said source being of such magnitude that said set I of waves of lower amplitude does not unblock said repeater, and means responsive to said set of waves of greater amplitude but not to said set of waves of lesser amplitude for unblocking said repeater.

12. A transmission system comprising two line sections over which signals are transmitted in both directions, a two-way repeater including two space discharge amplifiers having grid electrodes for repeating signals between said line sections, means for impressing a large biasing potential upon said grid electrodes to block said amplifiers, and means associated with each amplifier and responsive to incoming signals above a certain amplitude to reduce said grid potential to render said amplifier operative.

13. A transmission system comprising two line sections over which signals are trans; mitted in both directions, a two-way repeater including two space discharge amplifiers each having an anode, a cathode and a grid for repeating signals between said line sections, a Wheatstone bridge associated with each amplifier and includin a source of negative biasing potential for t e grid thereof, a space discharge device associated with each bridge having an anode, 'a cathode and a grid and having its cathode-anode resistance forming an arm of said brid e, and means for impressing incoming-sigma s upon each of said devices to vary the resistance of its plate circuit and thereby vary the biasing potential impressed upon the grid of the associated amplifier.

14. A transmission system comprising incoming and Outgoing lines over which speech and ringing currentsare transmitted, a space discharge amplifier having an anode, a cathode and a grid, an in ut circuit for saidamplifier, includingsai grid and said cathode line and an output circuit for said ampli er including said anode and said cathode and connected to said outgoing line, means for impressing a large negative biasing potential upon said grid to block said amplifier, means responsive to said speech and ringing currents to reduce said negative biasing potential to render said amplifier operative, a source of ringing current, and a relay in said output circuit operated 1n response to incoming ringing currents to connect said source to 

